[FFmpeg-devel] [PATCH] Add x86-optimized versions of exponent_min().
Fri Feb 4 00:47:12 CET 2011
On Thu, 3 Feb 2011, Justin Ruggles wrote:
> On 02/03/2011 12:05 AM, Loren Merritt wrote:
>> On Wed, 2 Feb 2011, Justin Ruggles wrote:
>>> Thanks for the suggestion. Below is a chart of the results for
>>> adding ALIGN 8 and ALIGN 16 before each of the 2 loops.
>>> LOOP1/LOOP2 MMX MMX2 SSE2
>>> NONE/NONE : 5270 5283 2757
>>> NONE/8 : 5200 5077 2644
>>> NONE/16 : 5723 3961 2161
>>> 8/NONE : 5214 5339 2787
>>> 8/8 : 5198* 5083 2722
>>> 8/16 : 5936 3902 2128
>>> 16/NONE : 6613 4788 2580
>>> 16/8 : 5490 3702 2020
>>> 16/16 : 5474 3680* 2000*
>> Other things that affect instruction size/count and therefore alignment
>> * compiling for x86_32 vs x86_64-unix vs win64
>> * register size (d vs q as per my previous patch)
>> * whether PIC is enabled (not relevant this time because this function
>> doesn't use any static consts)
> Doesn't yasm take these into account when using ALIGN?
ALIGN computes the number of NOPs to add, into order to result in some
address aligned by the requested amount. But that isn't necessarily
solving the right problem. If align16 is in some cases slower than align8,
then clearly it isn't just a case of being slow when it doesn't have
One possible cause of such effects is that which instructions are packed
into a 16byte aligned window affects the number of instructions that can
be decoded at once. This applies to every instruction everywhere (if
decoding is the bottleneck), not just at branch targets. Adding alignment
at one place can bump some later instruction across a decoding window, and
whether it does so depends on all of the size factors I mentioned.
>> * and sometimes not only the mod16 or mod64 alignment matters, but also
>> the difference in memory address between this function and the rest of the
>> While this isn't as bad as gcc's random code generator, don't assume
>> that the optimum you found in one configuration will be non-pessimal in
>> the others.
>> If there is a single optimal place to add a single optimal number of NOPs,
>> great. But often when I run into alignment weirdness, there is no such
>> solution, and the best I can do is poke it with a stick until I find some
>> combination of instructions that isn't so sensitive to alignment.
> I don't have much to poke around with as far as using different
> instructions in this case.
One stick to poke with is unrolling.
> So should we just accept what is an obvious bad case on one
> configuration because there is a chance that fixing it is worse
> in another?
My expectation of the effect of this fix on the performance of the
configurations you haven't benchmarked, is positive. If you don't want to
benchmark them, I won't reject this patch on those grounds.
I am merely saying that as long as you haven't identified the actual
cause of the slowdowns, as long as performance is still random unto you,
making decisions based on a thorough benchmark of only one compiler
configuration is generalizing from one data point.
> Even the worst case versions are 80-90% faster than the C version in the
> tested configuration (x86_64 unix). Is it likely that the worst case
> will be much slower in another?
Not more than 40% slower. (Some confidence since on this question your
benchmark counts as 24 data points, not 1.)
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